Typically, epoxy resins are converted from a liquid or thermoplastic state to thermoset solids by reactions involving the use of reactive compounds, or curing agents, which become a part of the crosslinked structure. Curing agents can be selected from a large number of reactive chemical structures but in practice a limited number of compounds are used in industry. The compounds most often used belong to one of the following six groups:
(1) aliphatic amines;
(2) polyamines;
(3) amine terminated polyamides;
(4) aromatic amines;
(5) anhydrides of dicarboxylic acids; and
(6) condensation products of formaldehyde with phenols or amides.
The reaction of diepoxides with diamines and polyamines is of primary commercial importance. Following is a schematic summary of the major reactions in the amine curing of epoxides. ##STR2##
The chemical reactions shown above compete for the epoxide groups in the system, their relative rates depending on the relative reactivities of specific reagents and intermediates. Reactions (1) and (2) predominate, the reactivity of the primary and secondary amine hydrogens depending on both induction and steric effects. ##STR3## are "amine terminated amides," the amine functionality being directly attached to the amide groups. As a result, the reactivity of these "amine" hydrogens is greatly affected by the strong inductive effect of the amide carbonyl. Hydrazides are easily synthesized from carboxylic acid esters and hydrazine hydrate.
Dihydrazides, like amines, can be used as curing agents in epoxide resin systems. Kamon and Saito, Kobunshi Ronbunshi 37, 765 (1980), Shikizai Kyokaishi 54, 416 (1981), and Kobunshi Kako 31, 10 (1982), have studied aliphatic carboxylic acid dihydrazides NH.sub.2 HNCO(CH.sub.2).sub.n CONHNH.sub.2 (n=2 to 18) as curing agents for epoxy resins. They found that the pot lives of these resin systems were much longer as compared to those in which aliphatic diamines were used, and that the curing mechanism of the terminal (primary) hydrogens of the hydrazides was similar to that observed for amines. The polar nature of the hydrazide bonds resulted in higher glass transition temperatures (T.sub.g) in the cured resins than in the case of aliphatic diamines.
Attempts have been made in the art to reuse crosslinked resins, but such attempts have not been successful. For example, epoxy resins have been converted to powders by pulverization or other mechanical means, and mixed with virgin resin for reuse in products of less demanding properties.
The art has heretofore failed to provide a method for crosslinking epoxy resins with a crosslinking or curing agent which can subsequently be cleaved and then, if desired, reformed, without significant sacrifice of the resin properties. Practically speaking, the commercial resins cannot be recovered and the constituents thereof, including structural fillers and the like, cannot be reused in new or modified resin structures.
It is the primary objective of this invention to convert cured epoxy resins to a polymeric material which can be reprocessed and crosslinked anew without impairing or significantly altering its properties.
It is a related purpose of this invention to provide curing compounds for epoxy resins which can be reversibly crosslinked without process inefficiency and without sacrificing the significant physical and chemical properties of the resin structure.
It is a further object of this invention to form cured epoxy resins wherein the crosslinks can be cleaved and reformed into useful new epoxy resins in an efficient and inexpensive manner.
It is a further object of this invention to form cured epoxy resins which can be reduced to a soluble form such that structured fillers (e.g., graphite fiber) can be efficiently removed from the resin for reuse.
It is still a further and significant object of this invention to employ curing agents which can react with the terminal epoxy groups of the resin to form the crosslinked network, which can subsequently be cleaved at a labile bond under mild reduction conditions and which can thereafter be modified into a variety of useful polymers.
It is still a further and related object of this invention to use crosslinking agents which are not expensive or difficult to produce and whose ability to be cleaved and reformed or cleaved and modified is such that the crosslinked resins can be efficiently recovered for reuse.